This invention relates to a coated metal substrate which absorbs solar energy selectively and to a method for coating metal substrates to provide a composite which absorbs solar energy selectively.
Metal bodies which selectively absorb solar thermal energy function in this fashion because of a high .alpha./.epsilon. ratio, wherein .alpha. is absorptance and .epsilon. is emittance. Metal bodies having requisite .alpha./.epsilon. ratios can be fabricated so that the heat absorbed is transferred to tubes carried in the metal body for subsequent use in heating and cooling operations.
Among coated metals which can be used for fabrication of selective solar thermal energy absorbers is aluminum. Coated aluminum absorbers are particularly valuable for heat exchange units because of the lightness of aluminum, with the resultant decrease in the complexity and weight of structual elements required for support thereof, and because of the ease with which aluminum can be machined and fabricated. See, for example, Lowery, U.S. Pat. No. 3,920,413, incorporated herein by reference.
McCoy, in U.S. Pat. No. 2,473,163, teaches that either bright or dull nickel can be plated on aluminum, following an anodic oxidation step to prepare the surface of the aluminum. However, McCoy does not indicate the sequential application of a bright nickel coating and a black nickel coating to an anodized or zinc coated aluminum base to produce a product having selective thermal absorptivity properties.
Peach, in U.S. Pat. No. 3,531,379, teaches a process for coating an aluminum object wherein a cathodic reduction step follows an anodic oxidation, but does not teach the preparation of a coating which is selective with respect to thermal absorption or emission properties.
Reinert, in U.S. Pat. No. 3,594,288, teaches coating aluminum with an adherent, wearable nickel surface, but does not suggest that the coating obtained exhibits any especially desirable thermal selectivity characteristics.
It has been found that the construction of Lowery, supra, tends to deteriorate with age and use, owing to instability to moisture over long periods of time.
Although black chrome is art recognized as a very stable absorber of solar thermal energy, a deficiency of black chrome is relatively high emissivity of the order of 0.12 and higher.
Although a solar thermal energy absorbing black oxide coating was obtained from thermal oxidation at 800.degree.-900.degree. F. of a bright nickel coating in accordance with Lowery, it the black oxide thus formed tended to delaminate from the aluminum substrate.
Thus, there is a continuing need for the development of solar thermal energy absorbing coatings on aluminum or other substrates which are stable to moisture over prolonged periods of time, which do not delaminate from the substrate under high temperature conditions, which have a low emissivity factor and which are economical and simple to prepare, even on a mass-production scale.